1. Field of the Invention
The present invention relates to a ring oscillator, and more particularly, a self-calibration circuit and method for calibrating phase offset between output waveforms of the ring oscillator.
2. Description of the Related Art
In general, for a mobile telecommunication system, since data is transmitted through two channels, i.e. an In-phase channel (I channel) and a Quadrature-phase channel (Q channel), both an In-phase (I) oscillating signal and a Quadrature-phase (Q) oscillating signal having a phase difference of 90 degrees each other are required to completely restore a desired signal. These oscillating signals are used as input signals of a down-converter for down-converting in frequency a signal received by a receiver to a baseband frequency at a receiving terminal of the receiver, and are used as input signals of a up-converter for up-converting the baseband signal to a signal of a higher frequency band at a transmitting terminal thereof. In the case where the In-phase oscillating signal and the Quadrature-phase oscillating signal do not have a correct 90 degrees phase, since a bit error rate is increased when restoring the desired signal lastly. Therefore, it is necessary that the I and Q oscillating signals having a correct phase difference each other be produced. Particularly, in the case of a direct conversion type receiver or an image rejection type receiver in which a signal is divided at a higher frequency through the two channels, i.e., the In-phase channel (I channel) and the Quadrature-phase channel (Q channel), an effect of an I-Q mismatch (i.e. a mismatch between the In-phase and the Quadrature-phase) is exhibited more seriously. For this reason, a correct operation of a transceiver always requires elimination of a phase error generated between the oscillating signals of the I and Q channels.
As a method of producing oscillating signals having a phase difference of 90 degrees each other, a method in which a phase shifter produces a signal having two phases by delaying an oscillating signal by a desired phase by using a resistor and a capacitor, and a method in which a quadrant generator with a master-slave construction produces oscillating signals having a phase difference of 90 degrees each other can be given. In the case of the former method employing the phase shifter, since there can occur a signal loss corresponding to a signal magnitude due to use of passive elements, an additional buffer circuit is generally required. Further, there still exists a problem of a mismatch between the I and Q oscillating signals due to a noise caused from the resistor, and a difference in the elements and signal paths.
Also, for the latter method for producing the I and Q oscillating signals, the quadrant generator with the master-slave construction is implemented by using two flip-flop circuits. In this case, an effect of a noise due to the passive elements is reduced, but an input signal having a frequency twice as large as frequencies of desired I and Q oscillating signals is necessary, which results in an increase of a required frequency of the oscillator. Also, in the case of employing the master-slave construction, when a duty cycle of the input signal does not become 50% precisely, there is generated a phase error between the I and Q oscillating signals. As a result, an additional circuit for matching the duty cycle of the input signal is needed.
However, in the case where a ring oscillator is employed to produce the I and Q oscillating signals of the transceiver, the I and Q oscillating signals having a phase difference of 90 degrees each other can be obtained easily even without using the phase shifter or the quadrant generator as mentioned above. The ring oscillator is a circuit designed to generate oscillation in such a fashion that a plurality of inverter circuits are basically connected to each other to compose a positive-feedback loop. Odd numbered stages are required to compose an oscillation feedback loop in a single-ended ring oscillator, but in a differential structured ring oscillator the oscillation feedback loop can be composed of even numbered stages.
In the ring oscillator, when the delay of a unit delay cell is Td, and the number of stages thereof is N, in the case where an even number of differential structured ring oscillators are used, since even numbered outputs are produced and two output signals having a phase difference of 180 degrees each other are generated from each delay cell, 2N output signals having a phase difference of 360 degrees/2N each other can be obtained. Accordingly, in case of implementing a ring oscillator with 2 stages, 4 stages, 8 stages, . . . , 2N stages, I and Q oscillating signals having a phase difference of 90 degrees each other can be obtained directly from an output of the ring oscillator without constructing a separate circuit, and it is possible to directly employ the I and Q oscillating signals in the transceiver.
However, even in case of the ring oscillator, there exists a phase offset between output waveforms having different phases because of a mismatch between delay elements constituting each stage of the ring oscillator or a difference in signal paths. For this reason, a phase difference between adjoining waveforms will be deviated from a value of the phase difference of 360 degrees/2N gradually. FIG. 1 is an output waveform of a ring oscillator for explaining a state in which a phase offset due to a mismatch between output waveforms occurs, wherein FIG. 1(a) is an output waveform of the ring oscillator in which the mismatch does not exist, and FIG. 1(b) is an output waveform of the ring oscillator in which the mismatch exists. Accordingly, in order to obtain the I and Q oscillating signals having a correct phase difference each other from the ring oscillator, there is needed a device and method for calibrating a phase offset between output waveforms caused due to a mismatch between the delay cells or a difference in signal paths.
Therefore, the present invention has been made in view of the above-mentioned problems, and it is an object of the present invention to provide a self-calibration device and method for eliminating a phase offset between output waveforms caused due to a mismatch between respective delay cells constituting a ring oscillator or a difference between signal paths.
Another object of the present invention is to provide a self-calibration device and method for producing I and Q oscillating signals having a correct phase difference each other from an output of a ring oscillator.
In order to achieve the above objects, according to an aspect of the present invention there is provided a self-calibration device for calibrating a phase difference between output waveforms of a ring oscillator, comprising:
a voltage-controlled oscillator adapted to adjust the transition time of an output signal according to an inputting of a control voltage for controlling the phase offset and generate the adjusted output signal;
a divider adapted to divide a frequency of the output signal generated from the voltage-controlled oscillator by a fractional number to generate a plurality of output waveforms having different phases with them having an identical phase difference each other;
a phase-locked loop (PLL) circuit adapted to correctly make a frequency and phase of the frequency-divided output signal generated from the divider coincident with those of a system clock, the phase-locked loop (PLL) circuit including at least a phase-frequency detecting means adapted to compare the frequency and phase of the frequency-divided output signal with those of the system clock and to output a result of the comparison; and
a phase offset calibrating loop circuit adapted to generate a control voltage for detecting a phase offset between output waveforms of the voltage-controlled oscillator and controlling the detected phase offset according to the result of the comparison inputted thereto from the phase-frequency detecting means for application to the voltage-controlled oscillator.
According to another aspect of the present invention there is also provided a method of calibrating a phase difference between output waveforms of a ring oscillator in a system including the ring oscillator, a divider adapted to divide a frequency of the output signal generated from the ring oscillator by a fractional number to generate a plurality of output waveforms having different phases with them having an identical phase difference each other, and a phase-locked loop (PLL) circuit adapted to correctly make a frequency and phase of the frequency-divided output signal coincident with those of a system clock, the phase-locked loop (PLL) circuit including at least a phase-frequency detecting means adapted to compare the frequency and phase of the frequency-divided output signal with those of the system clock and to output a result of the comparison, comprising the steps of:
determining whether or not the frequency-divided output signal generated from the divider and the system clock are locked;
detecting a phase offset of each of the delay cells constituting the ring oscillator according to the result of the comparison generated from the phase-frequency detecting means if it determined that the frequency-divided output signal and the system clock are locked; and
generating a control voltage for calibrating the detected phase offset to apply the generated control voltage to the ring oscillator so as to sequentially calibrate the phase offset for each of the delay cells.